1. Field of the Invention
The present invention relates to a system for switching and safeguarding data in time-division-multiplex switching networks in which both the two transmission paths between two neighboring time-division-multiplex exchanges, which paths are provided for one direction of transmission each, and the input circuit of one exchange, which input circuit is connected to the transmission paths and represents a synchronous group, are operated at the clock rate of said one exchange, and in which the input circuit lying opposite said input circuit and representing an asynchronous group is operated so that read and write operations to or from the transmission paths take place at the clock rate of said one exchange, while read and write operations to or from the switching grid of the other exchange take place at the clock rate of the other exchange.
2. Description of the Prior Art
Such an arrangement is disclosed in German Published Application (DT-OS) 2,048,734, corresponding to U.S. Pat. No. 3,725,590. The circuit arrangement shown there (FIG. 2) differs from conventional arrangements (FIG. 1) especially in that the "interfaces" between the different clock rates of the neighboring exchanges are combined in one exchange (asynchronous group), while the other exchange has no such interface (synchronous group). The difference between the clock rates used by neighboring exchanges results in so-called slip at the interfaces, i.e., if data to be switched arrive at an exchange at a clock rate higher than the clock rate of this exchange, incoming data must be suppressed in given time intervals and cannot be switched. Conversely, if the clock rate in the exchange is higher, individual items of the information arriving at a lower clock rate may be switched doubly. These errors may also occur in synchronous switching networks.
If the information to be switched in such a case consists of pulse-code-modulated speech, this slip has no effect within certain limits, because it does not endanger the intelligibility of the speech information to be switched. A few hundred slips per minute are considered still tolerable (see, for example, CCITT, Special Study Group D, COM Sp. D-No. 188 (1971). The trend, however, is towards also switching data through time-division-multiplex switching networks, even though telephone traffic is preponderant (see, for example, H. Pausch: "Developments and Plans . . . Germany", NTZ (1972), No. 9, pp. 416 to 418).
In the case of data, the problem of slip is more serious because the omission or addition of a data item falsifies this data on the whole and may even make it useless.